This invention relates to an assembly comprising a cylindrical container and a pressure control device for maintaining a constant predetermined pressure in the container which is arranged for dispensing a fluid contained in the container from the container at said pressure, the pressure control device comprising a first chamber and a second chamber as well as at least one closing member movable relative to the second chamber for releasing and closing a fluid connection between the first chamber and the container depending on the position of the closing member relative to the second chamber, the first chamber being filled with a gas which in use has a higher pressure than the pressure in the container, and the position of the closing member relative to the first chamber being at least dependent on the prevailing pressure in the container and the prevailing pressure in the second chamber, while in use the fluid connection is released when the pressure in the container decreases below the predetermined pressure, so that gas flows from the first chamber to the container and the pressure in the container increases again until the fluid connection is closed again by the closing member as a result of the increased pressure in the container, wherein the second chamber is located outside the first chamber, and the closing member comprises a stem which extends through a first opening of the first chamber wherein the first opening is provided with a sealing ring and the stem is provided with a circumferencial recess where the sealing ring extends in the recess. Such an assembly is known, from EP-A-0 446 973.
A disadvantage of the known assembly is that the fluid in the container will not always be suspended in a reliable and accurate manner. This means that with the passage of time, diffusion of gas from the first chamber and the second chamber to the container will occur. Since the pressure in the second chamber determines the predetermined pressure in the container of the container, this means that the predetermined pressure over time is unstable. In addition, it is found that the closing member will not react properly to a decreasing and increasing pressure in the container. In that event, accordingly, no accurate pressure control in the container will occur.
Furthermore the closing member is provided with a membrane to which the stem is attached for opening and closing of the first chamber.
This in turn has an adverse effect on the movability of the stem for regulating the pressure. So that pressure control becomes inaccurate This also results in the above disadvantage. Finally if the container is almost empty the effective volume of the container wherein the fluid to be dispensed is present becomes relative large to the volume of the fluid which may cause an inaccurate dispense of the fluid.
An assembly according to the invention is characterized in that the first chamber comprises a vessel which is provided with a first opening, the second chamber is formed by a cylinder which is closed at a first end and of which a second end constitutes a second opening and the closing member comprises a plunger moveable in the axial direction of the cylinder wherein the plunger is located into the cylinder and comprises the stem which extends from the vessel through the first opening of the vessel and the second opening of the cylinder to the cylinder the plunger is provided on its outer side with a sealing ring which constitutes a gas seal between an outer side of the plunger on one side and an inner side of the cylinder on the other, the vessel being designed as a plunger which is arranged to be received, in the container so as to be movable in axial direction of the container, the vessel dividing the container into an upper and a lower part, the fluid connection terminating in the lower part of the container, while the upper part of the container is filled with the fluid to be dispensed, and in use, when the pressure in the upper part) of the container decreases below the predetermined pressure, the pressure in the lower part of the container will likewise decrease because the vessel designed as a plunger will move such that the volume of the upper part of the container will decrease whereas the volume of the lower part of the container will increase, whereby also the fluid connection between the vessel and the lower part of the container is released, so that gas flows from the vessel to the lower part of the container and the pressure in the lower part of the container as well as the pressure in the upper part of the container will rise again, while the vessel designed as a plunger moves further upwards until the fluid connection is closed again by the closing member as a result of the increased pressure in the lower part of the container.
Because the first chamber comprises a vessel wherein the second chamber is located outside the vessel, the first chamber can be made completely gas tight.
Furthermore because the first chamber comprises a vessel it can easily be designed as a plunger which is arranged to be received, in use, in the container so as to be moveable in axial direction of the container. The container is then divided into an upper and lower part wherein the upper part of the container is filled with the fluid to be dispensed and wherein, in use, the fluid connection terminates in the lower part of the container. If almost all of the fluid in the container has been dispensed, the volume of the upper part will decrease, thereby guarantying an accurate and reliable and stable way of dispensing the residual liquid. Because the second chamber is located outside the vessel it can be formed by a cylinder comprising the plunger moveable in the axial direction of the cylinder. Because the plunger is moveable freely within the cylinder the stem which opens and closes the opening of the vessel responds only to the relevant gas pressures independent from the position of the plunger within the cylinder whereby again a highly accurate way of suspending fluid from the container is guaranteed. Because the outer part of the plunger is provided with a sealing ring a good gas seal between the outer side of the plunger on the one side and the innerside of the cylinder 10 on the other is obtained thereby avoiding that gas can flow from the upper part of the container to the lower part of the container and vice versa.
According to a particular embodiment, the volume of the second chamber is dependent on the position of the closing member relative to the second chamber. In that case, the prevailing pressure of the second chamber alone can be used for determining the predetermined pressure. It is also possible, however, that other means are included in the second chamber for the purpose of exerting a pressure on the closing member. To be considered here is, for instance, a spring.
According to a preferred embodiment, a sidewall of the first chamber is provided with a first opening through which the fluid connection extends, and a sidewall of the second chamber is provided with a second opening, the closing member extending from the first chamber via the first and second opening to the second chamber, a first subsurface of the closing member being situated in the first chamber and a second subsurface of the closing member being situated in the second chamber, the form of the first and second subsurface being such that a certain gas pressure which is exerted on the first subsurface results in a force in a direction of movement of the closing member that is smaller than the force in the direction of movement of the closing member that results when this gas pressure is exerted on the second subsurface, while the first opening can be released or closed by the closing member depending on the position of the closing member relative to the second chamber.